In many riveting applications using conventional, two-ended solid rivets, such as those used in aircraft assembly applications, one end of the rivet, which is inserted through two adjoining parts, is obscured from view by the configuration of one of the parts being assembled. For instance, in the assembly of an aircraft wing, involving the attachment of longitudinal stiffeners to a section of wing skin, one of the opposing riveting head assemblies, which includes a pressure foot and an anvil (ram), will move along the stiffener quite close to the leg portion thereof. The stiffener leg portion may take various configurations, including the particular "J" stringer configuration shown in FIG. 1.
Since, as illustrated in FIG. 1, the tolerance for movement of the riveting head assembly relative to the stringer is quite limited, except in the longitudinal direction, sometimes there will occur significant contact between the riveting head and the part assembly, i.e. the stringer, due to incorrect movement or erroneous initial positioning of the riveting head. This contact typically causes substantial damage to the assembly part as well as the riveting machine, resulting in significant loss in manufacturing time and considerable expense. Such contact and resulting damage is referred to generally as a "crash". Crashes, of course, are to be avoided if at all possible; they do, unfortunately, occur with regularity. This problem occurs for both the conventional hydraulic riveting machines, in which opposing riveting machine head assemblies are carried on a large "C" shaped yoke shown in FIG. 1, as well as for the newer electromagnetic riveting machines having separate, opposed riveting heads which are typically carried on a much lighter yoke, such as shown in U.S. Pat. No. 4,862,043 to Peter Zieve. The problem occurs most frequently in those riveting applications where one of the riveting heads is obscured from the view of the operator because of the configuration of one or more of the assembly parts, such as where one or more of the parts has a protruding and hooked configuration, like the combination of a wing panel and J stringer in FIG. 1.
Even relatively small errors in movement of the riveting heads during operation of the apparatus or in one or more of the several moves necessary to properly initially position the riveting heads will result in a crash in a confined operating situation. There have been attempts, however, to solve this significant problem. One such attempt includes the use of a riveting head pressure foot which releases during a crash. The pressure foot is held in position with a spring detent arrangement, and when a crash occurs the pressure foot assembly in effect breaks away and falls from the riveting head. The disadvantage of such an arrangement is that the operator must manually reinstall the pressure foot on the riveting head assembly after each such crash. This is time consuming and is often difficult since the riveting heads are frequently some distance off the ground. Hence, there is a significant need in many riveting applications to minimize and/or prevent damage which is caused by inadvertent contact between the riveting apparatus and the part assembly during riveting operations.
Another significant problem in many riveting applications involves the precise alignment of the two opposed riveting head assemblies on the rivet. It is important that each riveting head be aligned on the rivet to a relatively high tolerance, typically 0.002" or so, in order to ensure high quality rivet formation. The alignment of the riveting heads occurs in two directions, sideways, i.e. lengthwise along the part assembly and heightwise, i.e. vertically.
Typically, there is no difficulty aligning the one rivet head assembly which is on the same side of the panel as the drill unit, since accurate alignment of that assembly is achieved by simply moving the riveting head lengthwise the center distance between the riveting head and the drill. The riveting head on the drill side thus is centered on the rivet opening and thus centered on the rivet when the rivet is inserted therein.
For the opposing riveting head, the longitudinal alignment is typically accomplished by a conventional high resolution slide, such as a dovetail arrangement. However, the height of the riveting head is more difficult to control. In the past, shims have been used to set the desired height. However, this requires that the riveting head be first released from its mounting structure, the shims installed and then the riveting head remounted. This takes time and often is not very precise. Multiple shim insertions are usually required. Such a procedure cannot be accomplished during actual operation of the riveter.
Hence, there is a significant need for a height adjustment system for a riveting head assembly which is convenient and accurate and which can be used by the operator of the riveting apparatus. It would also be desirable to be able to accomplish the height adjustment during riveting operations.